Nonvolatile semiconductor memory devices such as flash memory can only perform a limited number of write and erase cycles before memory cells lose the ability to store data properly. Specifically, device operation generates defects, such as defects in the tunneling insulator, that trap charge, thereby degrading the ability of memory cells to store data. For example, a flash memory device may be limited to 10,000 write cycles or fewer. The time needed to program or erase a memory cell may also degrade with usage and the device is specified for the worst case characteristics.
Like reference numerals refer to corresponding parts throughout the drawings. For visual clarity and ease of description, cross-hatching has been omitted for various elements in the cross-sectional views.